Novel metamaterial concepts can be used to economically reduce flexural vibrations in coupled piperack systems. Here, we model pipe on flexible supports as periodic systems and formulate dispersion relations using Floquet-Bloch theory which is verified by a finite element model. Owing to the flexibility of the coupled system, a narrow pass band is created in low frequency regime, in contrast to the case of pipe without any rack. Two types of vibration reduction mechanisms are investigated for pipe with different supports, i.e. simple and elastic support. In order to tune the band gap behaviour, lateral localized resonators are attached at the centre of each unit cell; conversely, the lateral distributed resonators are realized with a secondary pipe existing in the system. The results reveal that both Bragg and resonance type band gaps coexist in piping systems due to the presence of spatial periodicity and local resonance. Although, the response attenuation of a coupled pipe-rack system with distributed resonators is found to be little lower than the case with the localized one, the relatively low stiffness and damping values lead to cheaper solutions. Therefore, the proposed concept of distributed resonators represents a promising application in piping, power and process industries. Pipes conveying fluid supported in equally spaced racks are very common in liquefied natural gas (LNG) plants, thermal power plants, petroleum industries, chemical plants and in many other engineering applications. LNG plant consists of many units such as gas receiving terminals, pipelines, storage tanks, etc. Long pipelines in such plants are used to carry refrigerated liquefied gas to storage tanks and shipping terminals. Excessive vibrations of pipelines due to ambient load, flow pulsation, valve or support excitation can result in fatigue damage, loosening of connections, etc., which may lead to fire, explosion, safety and environmental issues. It is thus essential to protect them from large vibration amplitude. To crystallize the idea, an LNG plant containing a coupled pipe-rack system connected to a tank 1 is shown in Fig. 1a. Such a system usually contains pipes of different dimensions supported on a finite periodic rack as highlighted in Fig. 1b. Periodic structures have been used as a common tool for mitigation of acoustic and elastic waves over the past decades 2-5. Periodicity in a structure may be in one, two or in all the three dimensions 6. Such systems exhibit unique frequency band gap characteristics 3 , which can be generated either due to the Bragg scattering 6,7 or by local resonances 8. As a result, they allow only waves of a certain frequencies to pass through, which are represented as pass or propagation bands. The remaining frequencies get attenuated, thereby forming stop or non-propagation bands. If the spatial periodicity of a structure is comparable to the wavelength λ, then Bragg band gaps are induced in the structure and appear around the frequencies governed by the Bragg condition = = … λ () l n n , where...

There are numerous ways to realize vibration absorbers. In this study, a new method is proposed wherein an elastomeric lattice is used. The geometrical configuration of the lattice is designed such that it transfers energy from the main system and is dissipated by the inherent material damping of the lattice material. The applicability of this system is numerically evaluated for pipelines by using two simple lattices whose geometries were optimized and the performances under harmonic loads compared with that of the theoretical optimal TMD. Eventhough they were capable of reducing the vibrations significantly, it was found to be less efficient at small mass ratios while at large mass ratios, the lattices performed similar to the theoretical optimal TMD. Nevertheless, in order to use such systems for pipelines or pipeline like structures such as chimneys, further studies are required using improved lattice configurations that can work efficiently for the whole range of mass ratios.

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Modal expansion techniques are typically used to expand the experimental modal displacements at sensor positions to all unmeasured degrees of freedom. Since in most cases, sensors can be attached only at limited locations in a structure, an expansion is essential to determine mode shapes, strains, stresses, etc. throughout the structure which can be used for structural health monitoring. Conventional sensor placement algorithms are mostly aimed to make the modal displacements at sensor positions of different modes as linearly independent as possible. However, under the presence of modelling errors and measurement noise, an optimal location based on this criterion is not guaranteed to provide an expanded mode shape which is close to the real mode shape. In this work, the expected value of normal distance between the real mode shape and the expanded mode shape is used as a measure of closeness between the two entities. Optimal sensor locations can be determined by minimizing this distance. This new criterion is applied on a simple cantilever beam and an industrial milling tower. In both cases, by using an exhaustive search of all possible sensor configurations it was possible to find sensor locations which resulted in a significant reduction in the distance when compared to a conventional optimal sensor placement strategy. Sufficiently accurate sub-optimal sequential sensor placement algorithm is also suggested as an alternative to the exhaustive search which is then compared with a genetic algorithmbased search. The efficiency of this new sensor placement criterion is further verified using Monte Carlo simulations for some realistic modelling error conditions.

Periodic structures exhibit unique band gap characteristics by virtue of which they behave as vibro-acoustic filters thereby allowing only waves within a certain frequency range to pass through. In this paper, both lateral and vertical flexural wave propagation and vibration control of a periodic railway track are studied in depth. More precisely, a rail fastened on rigid sleeper blocks is modeled with an Euler-Bernoulli beam. The dispersion relations in both lateral and vertical directions are obtained using the Floquet-Bloch theorem and the resulting dispersion curves are verified using finite element (FE) models. Afterwards, tuned mass dampers (TMDs) with different mass ratios are designed to control vibrations of the examined rail along both lateral and vertical directions. Moreover, the influence of damping of rail and resonators on band structures is investigated. As a replacement to the conventional TMD, a novel possibility to control vibrations relies on using another rail as a lateral distributed resonator (LDR). Although the effectiveness of LDR is lower than that of localized resonators, the former represents a simple and promising way to control vibrations. Efficacy of the proposed control methods is finally verified using the results of transient simulation based on a random Gaussian white noise input.